The present invention relates to an arc length changing apparatus of a color Braun tube exposing device and method thereof, and more particularly, by to an arc length changing apparatus in which the arc length of exposing device is variably changed to make the exposing quantity at the center portion and periphery portion of the panel constant so that a bending phenomenon of fluorescent film stripe formed on the internal surface of the panel is reduced.
Generally, three kinds of fluorescent material for emitting respectively the red, green and blue colors are painted in stripe form on the internal panel surface of a color Braun tube, and a black matrix made of light absorbing material is formed among said stripes of three colors of red, green and blue.
Red, green and blue fluorescent stripes as above described are formed by irradiating the light from a light source through fines holes of a shadow mask.
Structure of a general exposing device for irradiating the light from the light source, as shown in FIG. 1, is comprised of a lamp house 2 contained with a mercury lamp 1 as a light source, a shutter 3 shich is movable in right and left directions so that the light of said mercury lamp 1 is irradiated across a panel from one side therof to a central portion thereof and to the other side thereof. A compensation lens 4 is provided for correcting the refraction of light passing through said shutter 3, a filter 5 matches the rate of illumination intensity between the central panel portion and peripheral panel portions in response to the light irradiated to said panel 9. A shadow ring 6 limits the exposing area of the panel, and a head 7 supports the panel 9. The panel 9 mounts thereon a shadow mask 8 having a color discriminating function as a bridge role between panel 9 and light source.
The radiating length L of mercury lamp 1 becomes the arc length.
In the conventional exposing device comprised of such structure as above, light radiated from the mercury lamp 1 makes the photosensitive resin compositive material on the internal surface of panel 9 sensitized and thereby predetermined flouresent film is formed, wherein the sensitized portion on the internal surface of panel 9, as shown in FIG. 2A, is divided into portion A being of main-shadow and portion B being of semi-shadow according to the light quantity directed through any one hole of shadow mask 8, and semi-shadow portion B becomes darker than main-shadow portion A.
Therfore, in order that the same light quantity reaches said main-shadow portion A and semi-shadow portion B, as shown in FIG. 2B, another semi-shadow portion B' passed through adjoining hole of shadow mask 8 should be overlapped with said semi-shadow B. Arc length L corresponding to the length of said mercury lamp 1 should be selected to become such. When the selection of said arc length L is proper, fluorescent film of stripe form of clear straight line as shown in FIG. 3A is formed, but either when the arc length is long over necessity and irradiated light quantity is too much, or when it is short under necessity and irradiated light quantity is too small, as shown in FIGS. 3B and 3C, the stripe fluorescent film is not uniform and bending phenomenon occurs.
Further, since the irradiating light quantity becomes lee as going from the central portion of said panel 9 to the peripheral portion (refers to FIGS. 2A and 2B), in order to irradiate all portions with the same light quantity, it is necessary to make longer the arc length upon exposing the peripheral portion than the arc length upon exposing the central portion.
However, since it is impossible to change the length of the mercury lamp every time practically, heretofore, the mercury lamp having single arc length L has been used by selecting a pertinent intermediate value between the central portion and the peripheral portion of the panel, and therefore, there has been a problem that it was not possible to form the fluorescent film stripe of clear straight line.